This invention relates to a frequency response characteristic adjustable circuit particularly suited for a tone control circuit.
Tone control circuits used in audio amplifier systems usually have bass boost/cut and treble boost/cut frequency response characteristics. In order to obtain such frequency characteristics, various tone control circuits have been proposed, and they generally include such an RC circuit as shown in FIG. 1. The RC circuit of FIG. 1 is a basic bass-boost or treble-cut circuit comprised of resistors 1 and 3 and a capacitor 2 which has such a frequency response as shown in FIG. 2. The roll-off frequency fC1 and turn-over frequency fC2 of this circuit are respectively given by ##EQU1## where C1 is a capacitance value of the capacitor 2, and R1 and R2 are resistance values of the resistors 1 and 3 respectively. The adjustment of the attenuation level or circuit gain H over a frequency range above the frequency fC2 is usually made by adjusting either one of the resistors 1 and 3. However, this adjustment also changes the frequencies fC1 and fC2, that is, the frequency characteristic, as is seen from equations (1) and (2). In other words, the control circuit using the RC circuit shown in FIG. 1 has a drawback that adjustment of the attenuation level over a particular frequency range results in a change of the frequency range itself.
An object of the invention is to provide a frequency response characteristic adjustable circuit, which permits adjustment of the circuit gain over a particular frequency range without varying the frequency range itself.